Normally, the mass spectrometer uses plural ion transport optical elements to transport ions to a subsequent stage while suppressing dispersion of ions originating from sample. In a liquid chromatograph mass spectrometer (LC/MS) described in Patent Literature 1, for example, a desolvation pipe, an ion guide known as a Q array, a skimmer, an octupole ion guide, an entrance lens electrode, a pre-rod electrode, and the like disposed between an ion source and a quadrupole mass filter which is a mass spectrograph are all ion transport optical elements.
In a tandem quadrupole mass spectrometer (also known as a triple-quadrupole mass spectrometer), ions are fragmented by collision induced dissociation (CID) in a collision cell, and the generation efficiency of a targeted product ion by the fragmentation depends on the energy (collision energy) of an ion entering the collision cell. The collision energy depends on the voltages applied to the collision cell and the ion transport optical element in a preceding stage, and the ion intensity changes when the voltages are changed. Thus, it can be said that the collision cell is also an ion transport optical element in a broad sense.
The optimum voltage values of the ion transport optical elements that give maximum ion intensity slightly vary from spectrometer to spectrometer. Further, the optimum voltage values vary depending on the mass-to-charge ratio of a component to be analyzed. Therefore, in order to perform a high-sensitivity and high-accuracy analysis, an operation in search of the optimum voltage value to be applied to each ion transport optical element needs to be carried out at each mass-to-charge ratio on a spectrometer by spectrometer basis prior to an actual analysis. For this purpose, as described in Patent Literature 1, a conventional mass spectrometer is provided with an auto-tuning function to automatically determine an optimum value of the voltage to be applied to each ion transport optical element, which cancels an analyzer's burden of manually finding the optimum value of the applied voltage. It should be noted that a voltage value as referred to here means an amplitude value of the applied voltage when the applied voltage is a radio-frequency voltage, and means a voltage value itself when the applied voltage is a DC voltage.
When the applied voltage of each ion transport optical element is automatically adjusted using a typical conventional auto-tuning function, the applied voltage is incremented stepwise by a predetermined voltage step within a predetermined voltage range. At each voltage step, the ion intensity of the same component is observed, and a voltage value which maximizes or nearly maximizes the ion intensity is searched for. Normally, when the applied voltage of an ion transport optical element is changed, the ion intensity changes on a bell-shaped curve. Therefore, to find the voltage value which gives the maximum ion intensity, it is necessary to make the increment size (voltage step size) of the applied voltage as small as possible. However, naturally, the smaller the voltage step size is, the larger the number of measurement points becomes, requiring a longer time for automatic adjustment. If, on the other hand, the time for automatic adjustment should be reduced, the voltage step size needs to be increased, in which case the peak of the maximum ion intensity may be missed and the sensitivity is deteriorated.